Unveiling the neglected role of alcohols and phenolic compounds in photocatalysis H2O2 generation chemistry.

Alcohols are promising sacrificial agents for improving the photocatalytic production of hydrogen peroxide (H2O2) through reaction with holes, inhibiting the electron-hole recombination. However, the side effects of alcohols on oxygen reduction and reactive oxygen species (ROS) generation remain controversial due to intrinsic formation of organic peroxides under irradiation in the presence of air or oxygen. To address this, we employ electron paramagnetic resonance (EPR), in-situ Fourier-transform infrared spectroscopy (FTIR) and 1H nuclear magnetic resonance (NMR) to directly monitor intermediate species and eliminate false-positive results on H2O2 production. Our results demonstrate that varying oxygen concentrations, alcohol types, and alcohol dosages significantly influence H2O2 yields. Organic radicals were identified as critical intermediates, including hydroperoxyl radicals, alkoxyl radicals, and peroxyl radicals, contributing to H2O2 regeneration. Furthermore, the concept of sacrificial agents was expanded to phenolic compounds (PCs) and humic acid (HA) in the natural water environment. A strong linear correlation (R2= 0.91) was observed between the para-substituent Hammett constant (σp) of these "RHOH" structured PCs and the apparent rate constants of forming H2O2 (kf). Spectroscopy and molecular-level experiments were conducted to identify and quantify the generation of H2O2 and hydroxyl radical (•OH) during the HA photolysis. This study reveals an overlooked H2O2 re-form pathway in system containing sacrificial agents and provides valuable insights into synergistic mechanisms involved in photocatalytic of H2O2 generation and pollutant degradation, which was beneficial for the strategic development of the H2O2 energy generation synergistically water pollution treatment.